There’s one thing retiring former foreign minister Bob Carr and Liberal federal director Brian Loughnane agree on – the carbon tax dealt a blow to Labor at the 2013 election.

Mr Loughnane and Senator Carr, who will retire on Thursday, offered post-mortems on the election at separate events in Canberra on Wednesday.

Senator Carr, who was re-elected for a six-year term in September but announced on Wednesday he’d quit without serving a day, said Labor could win back government within one term under new leader Bill Shorten but needed a more cautious policy on climate change.

The carbon pricing issue had been mishandled and should have begun under Kevin Rudd in 2007 with a modest scheme applied only to the electricity sector’s output, he said.

“To have moved cautiously to start with, that … would have been a canny approach,” Senator Carr said.

The coalition government wants legislation to axe the carbon tax passed by Christmas, but faces opposition from Labor and the Greens in the Senate.

Mr Loughnane told the National Press Club the seeds of Labor’s election loss began with its deal with the Greens to form minority government in 2010, which included breaking its “no carbon tax” promise.

“In my view Labor did not need to introduce a carbon tax because the Greens ultimately would have continued to support them on the floor of the House,” Mr Loughnane said.

If Mr Shorten is serious about rebuilding from defeat he needed to be “honest enough to admit Labor got it wrong and support the important decisions which will have to be made” by the coalition government, Mr Loughnane said.

“I don’t believe Bill Shorten believes in a carbon tax.”

However, the fact that Mr Shorten, who hails from Labor’s Right faction, had to rely on support from the Left to win the leadership and did not win the grassroots majority in the ballot would compromise his authority.

“Labor has retreated to a mix of pre-Whitlam class-war prejudice and inner-city trendyism, overlaid by factional warlordism,” Mr Loughnane said.

The NSW Senate vacancy created by Senator Carr is expected to be filled by former Labor MP Deb O’Neill after a vote of the NSW ALP administrative committee.

Senator Carr revealed his decision to switch support from Julia Gillard to Kevin Rudd had come early in 2013 when he believed Labor was on track for a primary vote of as little as 25 per cent.

The final result was 33 per cent for Labor – the party’s lowest primary vote since 1903.

“I certainly thought the government had lost its way when, at the cabinet meeting convened to discuss coal seam gas, (which he had decided not to attend) there was a big package of media reforms dropped on the table,” Senator Carr said.

To take a fight up to the media six months from an election went against the rules of politics, the former NSW premier said.

Senator Carr said Mr Shorten, who he supported in the leadership ballot, would give Labor the best chance of getting back to government within three years.

“I believe that Bill Shorten and (deputy leader) Tanya Plibersek have got what it takes to get back to government in three years, a one-term strategy, or to get very close to it.”